The use of mechanical vibration produced at an ultrasonic frequency to weld thermoplastics, to clean and degrease materials and to emboss and form plastics is a well-established art in industrial processes (see, e.g., Ensminger, Ultrasonics, 1988, pp. 419-492). The physical principles underlying this technology have important relations to the invention described herein and therefore require review and discussion.
FIG. 1 shows an ultrasonic transducer 1 attached to a horn 2. The design and operation of such transducers, powered by piezoelectric crystals 3 is well known (see, e.g., Neppiras, Ultrasonics International 1973 Conf. Proc., pp. 295-302). To obtain significant vibrational motion, most systems are operated at one of their frequencies of extensional resonance. Both the transducer and horn are designed to resonate at the same frequency, in which case an extension, .epsilon., produced by the transducer at its ends is communicated to the horn. Since the horn is tuned to the same frequency as the transducer, it expands and contracts along its length in concert with the imposed motion. Because the overall length of the transducer and horn is an important consideration in practical equipment, the systems are usually designed to vibrate at their first mode of extensional resonance, for which the horn's length must be .GAMMA./2 where .GAMMA.=c/f is the velocity of sound in the horn material, and f is the chosen frequency of vibration. The motion produced at the free face of the horn is then reciprocal, or back and forth in the plane of the page, with an amplitude determined by the electrical voltage 4 applied to the transducer crystals. The diagram in FIG. 1 shows the variation 12 of extension, .epsilon., along the length of the structure.
Vibration of any rigid structure is always accompanied by stress. Because the piezoelectric crystals are fabricated from ceramic material they have a very limited tolerance to cyclic stress. For example, metals such as stainless steel or titanium can tolerate an indefinite number of cycles of tension and compression at levels from 10,000 to 40,000 psi. Ceramics, however, can not be exposed to cyclic stress much above 3,000 psi without the production of substantial internal heating and fracture. This limit is particularly pronounced when the stress is produced at ultrasonic frequencies (.gtoreq.18 kHz). In the half wave design shown in FIG. 1, the maximum stress occurs in the crystals, limiting therefore the maximum extension, .epsilon., to about 0.001 inch, p-p (peak to peak) at a frequency of 20 kHz. Very often practical applications require that the horn face vibrate with amplitudes in the range of 0.003 to 0.006 inch, p-p. For example, in the welding of polyester or polypropylene, vibration amplitudes of 0.003 to 0.0004 inch, p-p, are commonly used.
For applications where vibration amplitudes greater than those produced by the transducer itself are required, velocity transformers have been developed. FIG. 2 shows one such transformer or booster horn 5. This horn, which is interposed between the transducer 6 and the working horn 7, and serves to multiply the extension produced by the transducer by a number greater than one. Again, the transducer 6, booster horn 5 and working horn 7 are each designed to have the same resonant frequency. Many designs are available for such boosters and they have been thoroughly reviewed with respect to geometry and performance by others (see, e.g., Eisner, Physical Acoustics, 1964, pp. 353-363). The booster shown in FIG. 2 is a "stepped" horn in which two solids of constant, but different, cross sectional areas are joined in the region 13 of minimal extension.
It is also possible to incorporate amplification into the working horn, thus obviating the booster, as is shown in FIG. 3. The input extension 9 produced by the transducer 8 is amplified by the horn 10 to produce an increase in output extension at the working face 11. This horn 10 is also of the stepped design.
Ultrasonic vibration produced by the structures shown in FIGS. 1 to 3 have been commercially applied to continuous industrial processes in the performance of a variety of tasks. Such processes include laminating, wet or dry cleaning, atomizing, machining or drilling, rolling, densification, grinding, deburring, dewatering, soldering, welding, cutting and the like. The Emsinger text referred to above provides additional details of such processes which need not be repeated here.
The principle problem encountered in applying resonant ultrasonic vibration to these continuous processes is the limitation on horn width. Although the horn executes motion principally of contraction and extension, because the horn is invariably made of metal composed of molecules in a crystalline lattice it also expands and contracts in its width. FIG. 4 shows a rectangular structure subjected to a stress, .sigma., in the direction shown. In a resonator this stress will be a function of the vertical dimension, x. Hence .sigma., is shown as .sigma.(x). This stress produces a lateral contraction .delta. that is proportional to .sigma. and the lateral coordinate, w. The constant of proportionality is known as Poisson's Ratio.
FIG. 5 shows the form of motion along the perimeter of the horn that results from a combination of vertical and lateral motion. Normally, if the width of the horn is much smaller than its length, lateral contraction and dilation changes very little the direction and magnitude of the essentially vertical oscillation. However as the width approaches the length, lateral motion becomes substantial and begins to contribute a tangential component to the movement of the horn's surfaces Moreover, when the width is made equal to .GAMMA./2 lateral resonance occurs as shown in FIG. 6. Under this condition, the motion of the horn face varies from purely extensional at its center to velocity directed at a 45 degree angle to the face at its ends.
Tangential motion is usually not desired in welding, drying, cleaning or atomizing processes because in all such applications it is the component of motion perpendicular to the material that accomplishes the intended work. Hence, simple prismatic ultrasonic horns are limited in their width to a fraction of .GAMMA./2. For most metals, .GAMMA. is about 10 inches at a frequency of 20 kHz. Therefore, horn width must be less than 5 inches, and for the development of fairly uniform vibration amplitude across the horn face the width must be less than about 3 inches. While it is possible to increase .GAMMA. by lowering the frequency, once the frequency becomes audible (&lt;18 kHz) the incidental air borne sound emitted is hazardous to hearing. As a practical lower limit therefore, 18 kHz is used and 20 kHz has been established as the lowest safe operating frequency for most commercial high intensity ultrasonic equipment.
For economic reasons, process equipment is usually designed to make, finish or otherwise convert materials of widths from 60 to 360 inches. Because of horn width limitations, ultrasonic applications in the continuous process industries require the use of as many as 25 to 150 separate systems. In addition, each ultrasonic transducer, horn and ultrasonic generator must be adjusted to give substantially the same performance with respect to the working horn's output vibration in an attempt to obtain uniform treatment of the materials over their width.
To expand the applicability of ultrasonic vibration, a slotted horn is disclosed in U.S. Pat. No. 3,113,225. By introducing vertical slots at regular intervals along the horn's width, Poisson coupling is interrupted but extensional motion is conserved. The slotted horn was therefore a major advance in horn design. However, the introduction of slots imposed a severe requirement of other portions of the horn. FIG. 7 illustrates a slotted horn attached to a booster 43. The slots are placed at regular intervals .GAMMA./6 apart, giving an overall horn width of 7.GAMMA./6, or slightly more than one wavelength. The motion imposed at the booster connection 44 is purely extensional. Because the unslotted upper and lower regions of horn, shown crosshatched at 86, 87, are not perfectly rigid they do not communicate this motion faithfully to the other portions of the horn which are separated by the slots. Again, a tangential component of motion appears along the horn's edges as shown by arrows 45. The actual displacement of the horn is shown by envelopes for expansion 46 and contraction 47. Hence, although the slotted horn permits essentially extensional resonance to occur in horns having widths exceeding .GAMMA./2, it does not provide a uniform output across the horn face. Variations in the extensional component of motion in such horns are commonly observed to be on the order of 100 percent.
Attempts have been made to improve the output uniformity of slotted horns. Holze U.S. Pat. No. 4,315,181 stiffened the upper parts of the horn by increasing the thickness of the unslotted portions. Elbert et al. U.S. Pat. No. 4,607,185 added half wavelength resonators to the upper surface of the horn near the ends to enforce extensional motion at a point where it is otherwise most degraded. Harris et al. U.S. Pat. No. 4,651,043 introduced skewed slots, or slots placed at an angle to the vertical axis of a horn with integral amplification. These slots change the amplification of motion in the horn along its width and therefore compensate for dimunition of extension in the outboard regions. Welter U.S. Pat. No. 4,749,437 added masses to the upper surface of the horn to introduce inertial elements claimed to equalize output motion.
Despite these several modifications, it has been found impractical to build horns having substantially uniform output in widths greater than .GAMMA.. Consequently many industrial continuous process applications that could profit from the use of ultrasonic technology have found the investment in equipment to be prohibitively expensive and labor intensive in order to provide the process control apparatus which is necessary to ensure uniform station to station performance in an array. Mishiro U.S. Pat. No. 4,483,571 attempted to remove some of these obstacles to the use of ultrasonic processing by vibrating a solid bar with transducers attached at regular intervals along the bar's length. While this system does provide a single processing unit as opposed to an array, it does not provide uniform vibrational motion along the width of the bar, thus necessitating the use of at least two such systems, one offsetting the non-uniformity of the other, to ensure a mean uniform exposure to the processed material.
The principal problem encountered in extending the width of slotted horns, namely the finite flexural rigidity of the material above and below the slots, dooms the production of uniform extensional vibration beyond about .GAMMA.. Attempts to increase the flexural rigidity of the very sections that must continuously and faithfully transmit extensional motion introduces enough Poisson coupling in these thickened regions to again produce lateral motion. Thus, an extended width horn capable of providing uniform vibration over its width is provided by the preset invention to overcome the deficiencies of the prior art.
Such extended width horns would provide operational advantages in a wide variety of commercial processes, primarily in applications where material having an extended width must be subjected to ultrasonic vibration for a particular process purpose. Such advantages are obtained for processes which utilize the extended width horn in a stationary position over which the material would pass, or for situations when the extended width horn is moved across the surface of a material to be treated.
An example of a potential use for an extended width horn in a movable device is shown in U.S. Pat. No. 4,069,541, which discloses a liquid application and vacuum pickup cleaning apparatus utilizing ultrasonic vibration to agitate the cleaning solution. However, such extended width horns would be particularly useful in any continuous process for subjecting material to ultrasonic vibrations, including, for example, those processes described in U.S. Pat. Nos. 3,660,186, 4,044,174, 4,326,903, 4,605,454, 4,690,722, 4,713,132, 4,758,293, 4,770,730 and 4,823,783.
A major disadvantage as to the use of ultrasonic transducers for continuous processing relates to the removal of heat generated by the vibrating piezoelectric crystals. When crystals are mounted within two segments of material, as shown in FIGS. 1-3, the perimeter of the crystal must be exposed to the atmosphere in order to allow the air to cool the crystal. As such, the transducer is open to contact by moisture or fluid if used in a wet environment, with short circuiting or other damage to the crystal being possible.
A variation on the type of transducer of FIGS. 1-3 is known from U.S. Pat. No. 3,524,085. In this patent, the crystals are mounted at the end of the material to be vibrated, rather than at the center. This type transducer is preferred since it produces less heat for the same motional output compared to the transducer of FIGS. 1-3. Regarding the use of such a transducer in wet environments, the same problems mentioned above, although less severe, nevertheless apply. Thus, it would also be desirable to obtain a transducer which can be hermetically sealed or otherwise protected from moisture contacting the crystals while still retaining means for cooling the crystals.